1. Field of the Invention
This invention relates to novel polymeric compounds, polymers for polymer electrolytes comprising the same, and polymer electrolyte compositions having a high ionic conductivity.
2. Prior Art
Electrolytes used in secondary cells (batteries) and capacitors, for example, have up until now been primarily low-molecular-weight substances that are liquid at or above room temperature, such as water, ethylene carbonate, propylene carbonate, and tetrahydrofuran. In lithium-type cells in particular, use is commonly made of low-molecular-weight organic liquid electrolytes which tend to evaporate, ignite and burn rather easily. To ensure long-term stability, it is necessary to use a metal can as the outer cell enclosure and to increase the airtightness of the container. The result is a considerable rise in the weight of electrical and electronic components that use low-molecular-weight organic liquid electrolytes, and greater complexity of the production process.
Electrolytes can also be made of polymers. Such electrolytes have a very low volatility and thus are not prone to evaporation. Moreover, polymer electrolytes, as these are known, with a sufficiently high molecular weight can even be used as solid electrolytes that exhibit no fluidity at or above room temperature. They have the dual advantage of serving as a solvent for ion-conductive salts and of solidifying the electrolyte.
As an example of this type of polymer electrolyte, in 1978, Armond et al. at 1xe2x80x2 Universite de Grenoble in France discovered that lithium perchlorate dissolves in solid polyethylene oxide, and reported that when the concentration of 1 M lithium salt is dissolved in polyethylene oxide having a molecular weight of about 2,000, the resulting complex shows an ionic conductivity of about 10xe2x88x927 S/cm at room temperature. Other groups also reported that when the concentration of 1 M lithium salt is dissolved at room temperature in liquid polyethylene oxide having a molecular weight of about 200, the ionic conductivity at room temperature is about 10xe2x88x924 to 10xe2x88x925 S/cm. Thus, it is known that polymeric substances such as polyethylene oxide with the ability to dissolve ion-conductive salts function as electrolytes.
Since then, similar research has been carried out on a broad range of largely polyethylene oxide-related polymeric substances, such as polypropylene oxide, polyethyleneimine, polyurethanes and polyesters.
The most thoroughly investigated of these substances, polyethylene oxide, is a polymer having a good ability to dissolve ion-conductive salts as noted above, but at the same time, a semi-crystalline polymer. Because of the latter nature, when a large amount of metallic salt is dissolved in polyethylene oxide, the salt forms a pseudo-crosslinked structure between the polymer chains that leads to crystallization of the polymer. As a result, the ionic conductivity is much lower than predicted.
The reason is as follows. When an ion conductor is dissolved in a linear polyether-based polymer matrix such as polyethylene oxide, the ion conductor migrates, at temperatures above the glass transition temperature of the polymer matrix, through amorphous regions of the polymer matrix along with the local movement of polymer chain segments. Since the cations accounting for ionic conductivity are strongly coordinated by the polymer chains, the local movement of the polymer chains has a strong influence on cation mobility. The local movement of the polymer chains is called Brawnian motion.
Therefore, a linear polyether-based polymer such as polyethylene oxide is a poor choice as the matrix polymer for an ion-conductive polymer electrolyte. In fact, according to the literature to date, ion-conductive polymer electrolytes composed entirely of linear polymers such as polyethylene oxide, polypropylene oxide or polyethyleneimine generally have an ion conductivity at room temperature of about 10xe2x88x926 S/cm, and at best no more than about 10xe2x88x926 S/cm.
To obtain ion-conductive polymer electrolytes having a high conductivity, a molecule must be designed which allows the existence within the matrix polymer of many amorphous regions conducive to ion conductor mobility, and which does not crystallize even with the dissolution therein of ion-conductive salts to a high concentration. One such method is the attempt to introduce a branched structure into polyethylene oxide, as described in N. Ogata et al., Journal of the Japan Textile Society, pp. 52-57, 1990. Their work demonstrates that ion-conductive solid polymer electrolytes composed of a polyethylene oxide derivative having a high ionic conductivity (about 10xe2x88x924 S/cm at room temperature) can indeed be synthesized. However, commercialization of such polymer electrolytes has not been achieved due to the sheer complexity of the method of polymer synthesis involved.
There have also been reports on methods of attaining ion conductivity by imparting to the matrix polymer a three-dimensional network structure so as to impede the formation of a crystalline structure. One example of the use of a polymer having a three-dimensional network structure as the polymer matrix is a method of polymerizing an acrylic monomer or methacrylic monomer containing a polyoxyalkylene component as disclosed in JP-A 5-25353. This method, however, has a number of problems including the low solubility of the ion-conductive salt in the monomer, which necessitates the addition of a third component such as vinylene carbonate, and the low physical strength of the resulting polymer.
Therefore, one object of the present invention is to provide a novel polymeric compound, a polymer for a polymer electrolyte comprising the same, and a polymer electrolyte composition having a high ionic conductivity.
The inventor has found that a novel polymeric compound comprising units of the formulas (1) and (2) to be defined below, wherein at least 10% of the ends of molecular chains are capped with one or more substituents selected from the group consisting of halogen atoms, substituted or unsubstituted monovalent hydrocarbon groups, R1COxe2x80x94 groups (wherein R1 is a substituted or unsubstituted monovalent hydrocarbon group), R13Sixe2x80x94 groups (wherein R1 is as defined above), amino groups, alkylamino groups, H(OR2)mxe2x80x94 groups (wherein R2 is an alkylene group having 2 to 4 carbons, and letter m is an integer of 1 to 100), and phosphorus-containing groups, and especially with cyano-substituted monovalent hydrocarbon groups or a mixture of cyano-substituted monovalent hydrocarbon groups and R13Sixe2x80x94groups, has a high electrochemical stability. The polymeric compound also has an ability to dissolve an ion-conductive salt to a high concentration, and does not crystallize and remains amorphous even with the dissolution of an ion-conductive salt to a high concentration. This means that the polymeric compound is an amorphous polymer ensuring the free mobility of an ion conductor therethrough. It has also been found that a composition comprising an ion-conductive salt dissolved in the polymer in high concentrations exhibits an excellent ionic conductivity and is best suited as a polymer electrolyte in a lithium-type secondary cell.
Accordingly, the invention provides a polymeric compound comprising a unit of the following formula (1) and a unit of the following formula (2). 
At least 10% of the ends of molecular chains are capped with monovalent groups of at least one type selected from the group consisting of halogen atoms, substituted or unsubstituted monovalent hydrocarbon groups, R1COxe2x80x94 groups, R13Sixe2x80x94 groups, amino groups, alkylamino groups, H(OR2)mxe2x80x94 groups, and phosphorus-containing groups wherein R1 is a substituted or unsubstituted monovalent hydrocarbon group, R2 is an alkylene group having 2 to 4 carbons, and the letter m is an integer from 1 to 100.
In a second aspect, the invention provides a polymer for a polymer electrolyte comprising the polymeric compound defined above. In a third aspect, the invention provides an ion-conductive polymer electrolyte composition primarily comprising the polymer for a polymer electrolyte and an ion-conductive salt.